Brushless fans are utilized in, for instance, computers, desktop fans, bathroom fans, and ventilation fans, and the like, because they are not as noisy as those operated by brushes. Nevertheless, brushless fan motors when operated, and when the stator energized electro-magnetic coils change polarization, an unwanted clicking or snapping noise can be heard particularly when the revolving fan blades do not override the clicking noise made by the fan motor. It is sometimes important that ventilation fans are as quiet as possible and the clicking noise should therefore be suppressed to avoid the noise problem.
A brushless fan motor can be driven by direct current (DC) by feeding DC to driver transistors through which the coils are energized. One object of the present invention is to provide a brushless DC motor, which reduces or suppresses the clicking/snapping noise produced when the motor switches its magnetic poles during operation. One important feature of the present invention is the concept of controlling the revolutions per minute (rpm) of the motor through pulse width modulated (PWM) signals when switching transistors and the controlling electronics are integrated inside the motor. The correct timing of the switching of the transistors may be accomplished by using a magnetic sensor, as described in detail below. For example, the on-time of the duty cycle of the pulsating PWM signal may be reduced to reduce the energy/voltage that in turn reduces the rpm of the motor. Thus, when a motor is not controlled through PWM signals, the motor obtains the rpm provided by the voltage when the magnetic circuit alters windings. When the rpm of the motor is only controlled by the variation of the voltage, the motor becomes weak when operating at a low rpm. The control of the rpm with the rpm feedback signal of the present invention makes the motor strong even at a low rpm. A first filter or pulse truncating device is used to truncate the up going flank of the pulsating PWM signals to softly or gradually open a first transistor. The smooth opening of the driving transistor reduces the noise from the stator coils during the switching on or turning-on process. The first filter ensures that the PWM pulses have a rise time as a result of the truncation, which allows the transistors to open smoothly and thus more quietly. To clarify, the first filter is not used to filter out the PWM pulses but is used to suppress undesirable noise by slowing down the rise time of the PWM pulses so that the transistors open more smoothly. It was surprisingly discovered that a very rapid opening of the transistors creates an undesirable clicking noise at the coils and that this clicking noise can be suppressed by truncating the PWM pulses to include a soft rise time which, in turn, permits the transistors to open more slowly to avoid the clicking noise from the coils. A transistor is similar to a water faucet in that if it is opened gradually or smoothly then this avoids the undesirable pressure peaks which cause the banging noise in the water pipe. In this way, the transistors of the present invention are gradually or slowly opened for each PWM pulse coming to the transistor gate by truncating the PWM pulses with a rise time which then allows the transistors to open up a bit slower. It should be understood that the words “gradually” and “slowly” are relative terms and merely mean that the transistors open more slowly compared to how the transistors would open if the PWM pulse had not been truncated. The opening of the transistors is still a fast process since they open in micro- or nano-seconds depending upon the frequency of the PWM signal. When the PWM frequency is about 16 kHz, the transistors may open between 1-5 microseconds, more preferably about 3 microseconds, softer or more gradual compared to the more sudden increase of the current when un-truncated signals are used. Rapid openings of the transistors, i.e. when the PWM pulses are not truncated, results in the sudden flow of current through the stator windings, which creates the undesirable clicking noise in the stator windings of the coils. By making the current-increase into the stator windings more gradual or slower, the current-increase is somewhat longer so there is enough time to start up the creation of magnetic fields in the stator without also creating the undesirable clicking sound. When the PWM pulses are truncated, the current-increase is still very fast, but slightly slower than if only the inductance limits the increasing of the current. The truncated signal may slow down the opening of the transistors with 150 nanoseconds to about 10 microseconds. The shorter time applies to high PWM frequencies. If only the coil inductance is used to limit the increase of the current, then the current rushes into the coils too fast and causes the undesirable clicking noise in the stator windings. In general, the coil inductance resists rapid changes of the current and the level of the resistance partly depends on the coil inductance. In the present invention, it is desirable to slow down the current increase through the coils more than what can be accomplished by merely relying on the slowdown caused by coil inductance.
A second filter or transient suppressor has a non-polarized capacitor that may be used to suppress noise caused by transients induced in the stator windings when the current is interrupted by a switch of transistors as a result of the PWM signal being sent into the second transistor instead of into the first transistor. A magnetic sensor may be used to sense when the direction of the magnetic field is switched (i.e. the polarity is switched from south pole to north pole or vice versa) at a fixed position relative to the stator so that it may be determined which transistor should be activated. In other words, the signal from the magnetic sensor may be mixed with the PWM signal so that the PWM signal is sent to the correct transistor at the right moment. One important feature of using non-polarized capacitor is that the noise from transients is suppressed and absorbed by electrolyte in the capacitor without creating any counter-acting force. Preferably, the turning-off of the first transistor should occur quickly so that the current-flow is quickly interrupted and so that the second transistor can draw current flow in the opposite direction. In a way, the electrolyte of the non-polarized capacitor absorbs the transient energy from the first winding when the current to the first transistor is abruptly turned off. When the second transistor starts conducting at the same moment the first transistor stops conducting, then the polarity of the non-polarized capacitor is switched and the capacitor charge changes polarity.
The acceptable level of the voltage transients may be set by using diodes and zener-diodes. It was surprisingly discovered that the second filter may thus be used to reduce the transient noise from the winding when the transistors are switched i.e. so that one transistor is switched on while another transistor is switched off. The use of the first and second filters to reduce the noise level from the motor during operation is particularly important when the rpm of the motor is relatively low so that the noise level from the motor is louder than the noise level from the rotating fan blades. The noise reducing features of the two filters may, of course, be used although the noise from the rotating fan blades exceeds that of the operating motor.
Preferably, the direct current motor of the present invention has a stator with at least four poles constituted by at least four teeth/arms. Each tooth has two electromagnetic coils making up a magnetic north and south-pole when energized with current-flow in the opposite direction each time. The motor should have one rotor constituted by at least two static magnetic north poles and two magnetic south poles. The motor of the present invention may have at least one of a central processor (CPU), or an electronic circuit unit that may be used to generate the PWM signal at a frequency range that is difficult to hear by a human ear. Preferably, the signal is adapted to be transmitted as a first input signal to a double gate function performing two AND gate functions so that each provides a PWM signal to each one of the two coil driving transistors in order to magnetize the coils in the four stator poles as north and south electromagnetic poles, respectively. Preferably, the stator thus has four poles that are wound so that every other winding is alternatingly wound in an opposite direction. The winding arrangement includes two parallel windings. This means that when current flows through one of the windings two south and two north poles are formed and when the current flows in the other winding all four poles change magnetic polarity so that north becomes south and south becomes north. The AND function gates are, preferably, adapted to receive a second signal activated by a sensor that measures a change in polarization of the rotors magnetic field in relation to the stator so that the AND function gates send the PWM signal to the driving transistors at the right time. The current through the coils may be determined by the duty cycle of the PWM signal and the power voltage to the windings and this determines the power of the created magnetic field in the stator poles. The PWM signal is, preferably, adapted to be received by at least two driving transistors so that each transistor may receive the PWM modulated signal from each of the AND gates. In this way, each driving transistor may receive an alternated modulated signal, based on the position of the magnetic field of the rotors to energize the coils into a north and south-pole at the right moment, respectively and based on the alternation of the magnetic field of the rotors. The driving transistors drive the coils every second time and alternate the current direction through the coils. Preferably, the first filter is connected to the gates of the driving transistors to ground. The time constant of the first filter may be determined by the frequency of the PWM signal. In this way, the first filter may be designed to open the transistors softly/smoothly. As indicated above, this ensures that the PWM pulses have a rise time which allows the transistors opens smoothly. The average current through the transistors decreases for each PWM pulse with increasing filter time. The second filter is, preferably, connected between the wire coils which have at least one capacitor that creates a non-polarized electrolyte which may be used to suppress the snapping sound from the motor when the transistors open up or close i.e. when the current in the stator coil is switched from one transistor to another transistor.
In one embodiment, the non-polarized capacitor is constituted by two serial connected electrolytic capacitors with altered polarity making up the second filter. Another embodiment provides that there is a ceramic capacitor connected in parallel included in the second filter.
Another embodiment provides that the zener diode is connected over each of the coil windings in series with a diode mounted in a reverse direction to suppress voltage transients in the zener direction so that the transients are limited to the level of the zener voltage plus the forward voltage drop of the diode. In yet another embodiment, the PWM signal is generated and sent from the central processing unit or a PWM circuit. Still yet another embodiment provides that the AND function gates are provided with the signal from the magnetic field sensor itself. The AND gate function can also be integrated inside the CPU.
Further one embodiment provides that a predetermined higher voltage output is utilized in a switch function to disconnect the first and the second filter as soon as the noise from the motors load overrides the noise from the motor itself. This higher voltage across the zener diode plus the diode can be used to power up a circuitry that can be used to disconnect the capacitors in the second filter with N-FET transistors when the noise level of the fan blades is so loud that the second filter is not needed since the noise from the fan blades is louder than any noise from the coils. This circuit may be controlled by a control signal from the CPU or the electronic circuit to control when it should connect or disconnect the capacitor based upon the rpm of the motor.
This higher voltage can provide a circuit with driving voltage so that N-FET transistors can work as an analog switch which may be used to disconnect the capacitors in the second filter when the noise level of the fan blades, as determined by the rpm of the motor, is so high that the filter does not need to suppress the clicking noise. In this way, a transistor can cut off the second filter through a signal from the microcontroller or the electronic circuit. It is important to distinguish these transistors from the other driving transistors that are in operative engagement with the windings.
A still further embodiment provides that an input signal is sent to the central processing unit or an electronic circuit through the sensor determining at which rpm the motor is operating. The sensor thus determines how fast the motor rotates but it does not necessarily determine at what rpm the motor should rotate. Preferably, this magnetic sensor provides information about when the polarity of the magnetic field in the rotor is changed and the sensor is positioned in a fixed place relative to the stator. This sensor may have a built-in complementary output that is connected to the inputs at each AND gates and they may be used to send the PWM signals to the coil driving transistors when the magnetic sensor senses a switch from a north to a south magnetic field or vice versa. For example, this means that if the sensor senses a switch to a south magnetic field, one AND gate opens up and sends the PWM signal to the first driving transistor. If the sensor senses a north magnetic field, the other AND gate opens up and sends the PWM signal to the second driving transistor. It is to be understood that there are different ways of generating the PWM signal. For example, if separate AND gates are used, the CPU or separate electronics generates the PWM signal that is sent to the AND gates where it is AND connected together with signals from the magnetic sensor. If the signals from the magnetic sensor are sent directly to the CPU and the AND function is integrated into the software of the CPU, the CPU may generate the PWM signals and send them directly to the correct transistor. Preferably, the AND gate is set up so that when the AND gate has two input gates and both receive signals, the signal received in the first input gate is passed through the output gate of the AND gate. In other words, if the AND gate only receives a PWM signal on the input gate then nothing is passed through to the output gate. However, when the input gate receives the PWM signal and the second input gate receives the signal from the magnetic sensor, then the AND gate opens the output gate so that the PWM signal may pass through the output gate of the AND gate i.e. as long as the magnetic sensor senses the magnetic field and generates the signal to the second input gate of the AND gate. It is also possible to use the pulses from the magnetic sensor to determine the rpm of the motor and when the rpm is known it is possible to adjust the PWM signal to a desirable rpm by changing the duty cycle of the PWM pulse to increase or reduce the rpm of the motor. This embodiment provides that the motor may be controlled through revolution feedback control with this sensor as an RPM input signal.
In summary, the method of the present invention is for making the brushless direct current (DC) motor quieter. The predetermined high frequency pulse width modulated (PWM) signal is generated. The PWM signal is sent to a first filter. The first filter truncates the PWM signal to provide the PWM signal with a longer rise time. The rise time allows transistors connected thereto to open smoothly. A second filter is provided that has a non-polarized capacitor. In a coil switching process, the non-polarized capacitor operates as a voltage or current absorption circuit between driving transistors. The coil switching process creates transient energy of voltage transients. The non-polarized capacitor absorbs the transient energy.
More particularly, the PWM signal is generated through at least one of a central processing unit, or an electronic circuit. The PWM signal is transmitted as a first input signal to an AND gate function. The AND gate function performs an AND gate function with the PWM signal and the signal from the magnetic field sensor. Each AND function gate provides an output signal to each one of the coil driving transistors to magnetize coils as north and south electromagnetic poles, respectively. A second input signal is sent to the AND gates, and that signal is activated by the magnetic field sensor when a change in the polarization of a magnetic field of rotors is measured the AND gate opens and puts out the PWM signal. It should be understood that the use of four poles for the motor is merely an illustrative example and that it is possible to use more or fewer poles if necessary.
The opening of the output gate of the AND gate occurs when the AND gate receives an input signal from the magnetic field sensor and the PWM signal as an input signal at the other input gate of the AND gate. Preferably, at least two driving transistors receive the PWM signal. Each driving transistor receives the PWM signal from one of the AND function gates. Each transistor receives a PWM signal, based on the polarity of the magnetic field of the rotors at the sensor position to energize the stator coils into a north and south magnetic pole, respectively, based on alternation of the magnetic field of the rotors. The driving transistors drive the stator coils every second time alternating a current direction and current strength through the stator coils. The first filter is, preferably, connected between the outputs of the AND gates and the gates of the driving transistors. A time constant of the first filter may be determined by a frequency of the PWM signal. The first filter is adapted to open the driving transistors to ensure that pulses of the PWM signal have a rise time which allows the driving transistors to opens smoothly. The average current through the driving transistors decreases with increasing filter time of the first filter. The second filter is, preferably, connected between the coils. The second filter has a capacitor that suppresses a snapping sound from the motor when the driving transistors open up and stop conducting current.
Preferably, the capacitor is a non-polarized electrolytic capacitor. The capacitor may be provided as two serial connected electrolytic capacitors with the same polarity connected to each other. For example, the plus poles of the capacitors are connected together or the minus poles of the capacitors are connected together to create a non-polarized capacitor. The other sides of the serial-connected capacitors are connected to the coils. This connection may be illustrated as −++− or +−−+.
The capacitor may be connected in parallel with a ceramic capacitor. Also, the zener diode may be connected over each of the coil windings in series with a diode mounted in reverse direction to suppress voltage transients in the zener direction and limiting the transients to a level of a voltage of the zener diode plus a forward voltage drop of the diode. A bi-directional TVS diode (Transient Voltage Suppressor) may also be used.
The method of the present invention for making a motor quieter includes the step of generating a predetermined high frequency pulse width modulated (PWM) signal. The PWM signal is sent to a first filter. The first filter truncates the PWM signal to provide the PWM signal with a rise time. The rise time allows driving transistors connected to the first filter to open smoothly. The second filter has a non-polarized capacitor. In a coil switching process, the non-polarized capacitor operates as a voltage and/or current absorption circuit between the driving transistors. The coil switching process creates transient energy of voltage transients. The non-polarized capacitor absorbs the transient energy. A CPU creates and sends the PWM signal, A magnetic sensor is provided that is moved to permit the CPU to delay sending the PWM signal. The CPU is provided with software to permit the CPU to delay sending the PWM signal. The current and voltage of the motor is measured to determine a motor load. A motor effect of the motor is determined based on the current and voltage of the motor. An efficiency of the motor is improved by moving a switching point for activating the stator windings. The PWM signal is used to control revolutions per minute (rpm) of the motor. The magnetic sensor sends a signal to a first input of an AND gate and a CPU sending the PWM signal to a second input of the AND gate. The non-polarized capacitor suppresses noise from the voltage transients. Zener diodes are used to limit the voltage transients. The CPU disconnects the non-polarized capacitor when revolutions per minute (rpm) of the motor are above a threshold value. In other words, when the sound of the fan is louder than the electrical sound from the motor, both the first and the second filters may be turned off to improve the efficiency of the motor.
The method is directed to making a direct current motor quieter by generating a predetermined high frequency pulse width modulated (PWM) signal through at least one of a central processing unit, or an electronic circuit, transmitting the PWM signal as a first input signal to a double gate function. The double gate function performs two AND gate functions at AND function gates. Each AND function gate provides a signal to each one of coil driving transistors to magnetize coils as north and south electromagnetic poles, respectively. A second signal, that is activated by a magnetic field sensor, is sent to the AND function gates. A change in polarization of a magnetic field of rotors is measured. A magnetic field sensor opens the AND function gates and outputs the PWM signal. At least two driving transistors receive the PWM signal. Each driving transistor receives the PWM signal from one of the AND function. Each transistor receives an alternated modulated signal, based on the magnetic field of the rotors to energize the coils into a north and south magnetic pole, respectively, based on alternation of the magnetic field of the rotors. The driving transistors drive the coils every second time alternating a current direction and current strength through the coils. A first filter is connected to the AND function gates of the driving transistors. A time constant of the first filter is determined by a frequency of the PWM signal. The first filter is adapted to open the driving transistors and ensures that pulses of the PWM have a rise time which allows the driving transistors to opens smoothly. The average current through the driving transistors decreases with increasing filter time of the first filter. The first filter may be used to control the current but most of the control and adjustments of the current is preferably done via the PWM signal and its duty cycle. A second filter is connected between the coils. The second filter has a capacitor. The capacitor suppresses a snapping sound from the motor when the driving transistors open up and close while drawing current through the coils to ground. The capacitor is provided as a non-polarized electrolytic capacitor. The capacitor is connected in parallel with a ceramic capacitor. A zener diode is connected over each of the coil windings in series with a diode mounted in reverse direction to suppress voltage transients in the zener direction and limits the transients to a level of a voltage of the zener diode plus a forward voltage drop of the diode. The central processing unit sends the second signal to the AND function gates. The magnetic sensor sends a signal to the CPU about a position of the rotor. The CPU creates the PWM signal and sends the PWM signal. The motor is controlled through a revolution feedback control.
The method for making a motor quieter includes the step of sending a pulse width modulated (PWM) signal to a first transistor. The first transistor conducts current into a stator winding. A second transistor is switched to by sending the PWM signal to the second transistor instead to the first transistor. The first transistor terminates conduction of the current into the stator winding and induces transients in the stator winding. A capacitor suppresses the transients induced by the first transistor. A non-polarized electrolyte capacitor is used to suppress noise from transients and a diode to set a voltage limit of the capacitor. A zener diode sets a voltage limit of the non-polarized electrolyte capacitor.
The method for making a motor quieter includes the step of a first transistor conducting current in a first direction through a stator winding and a capacitor. The first transistor is de-activated and a second transistor is activated. The second transistor conducts current in a second direction through the stator winding and the capacitor. The capacitor absorbs transient energy from the current created in a time period between deactivation of the first transistor and activation of the second transistor. A first end of the capacitor is filled with transient energy in the first direction while emptying previously stored transient energy out through a second end of the capacitor. The previously stored transient energy is emptied in the first direction.
The method is for making an electric motor more efficient. An electric motor is provided that has a rotor being rotatable in a first direction relative to a stator, a first and a second transistor electrically connected to the stator and to a processor. The rotor has magnets of first and second polarities separated at polarity changing points. A sensor senses a first polarity changing point. The sensor sends a first triggering signal to the processor. Upon receipt of the first triggering signal, the processor delays by a time period (t1) before sending a first activation signal to the first transistor to start rotating the rotor in the first direction. The first activation signal lasts for a time period (I1). The processor measures a current A1 driving the electric motor at a rotational speed. The sensor senses a second polarity changing point and sends a second triggering signal to the processor. Upon receipt of the second triggering signal, the processor delays by a time period (t1′) before sending a second activation signal to the second transistor to continue rotating the rotor in the first direction. The second activation signal lasts for a time period (I2). The processor measures a current A2 driving the electric motor at the rotational speed. The processor compares the current A1 to the current A2 and selects time period (t1) for sending activation signals when the current A2 is greater than the current A1 and selects time period (t1′) for sending activation signals when the current A1 is greater than the current A2. The processor iteratively changes the time period (t1) for each activation signal sent until a minimum current Amin is found by comparing measured currents to optimize an efficiency of the electric motor.
In another embodiment, a first PWM pulse is used as the first activation signal.
A length of the first PWM pulse to the first transistor is varied.
The current A1 and the current A2 are measured at a constant rotational speed (ω) of the rotor.
The processor continuously monitors currents driving the electric motor.
The first and second transistors are alternatingly used to drive the rotor in the first rotational direction.
The method is also for changing a rotational direction of a rotor of an electric motor. An electric motor is provided that has a rotor rotatable in a first direction and in a second opposite direction relative to a stator. A first and a second transistor are electrically connected to the stator and to a processor. The rotor has magnets of first and second polarities separated at polarity changing points. A first magnetic sensor is located (α1) degrees prior to an activation point when the rotor rotates in the first rotational direction and a second magnetic sensor is located (α2) degrees prior to an activation point when the rotor rotates in the second opposite rotational direction. The first magnetic sensor senses a first change of polarity at a first polarity changing point on the rotor and sends a first triggering signal to the processor. After receipt of the first triggering signal, the processor sends a first activation signal to the first transistor to keep on rotating the rotor in the first rotational direction. The processor receives a change of rotation command. The processor sends a second activation signal to the second transistor before sending any activation signal to the first transistor to rotate the rotator in the second opposite rotational direction. The second magnetic sensor senses a second change of polarity at a second polarity changing point on the rotor and sends a second triggering signal to the processor. After receipt of the second triggering signal, the processor sends a third activation signal to the first transistor to keep on rotating the rotor in the second opposite rotational direction.
In another embodiment, a first voltage interval is used to characterize the first rotational direction and a second voltage interval to characterize the second rotational direction.